Pelagic animals live in the open seas, away from the coast or seafloor. The Pelagic Ecosystem team has the task of studying these predator and prey species in Prince William Sound. Despite the challenge, scientists have already managed to collect decades of data that focus on the interactions between whales, seabirds and their prey.

This information is useful in answering questions such as:

• What are the population trends of key open-ocean predators, such as orcas, tufted puffins, and humpback whales?

• Are the numbers of forage fish, like herring, sand lance, and capelin, going up or down?

• Is it possible to monitor forage fish population trends?

• If it is possible to monitor them, what is the best way to do so?

Forage fish have a big impact on marine ecosystems. They convert a huge amount of energy from lower trophic levels and this energy is transferred into food for larger fish, marine mammals, and seabirds. Forage fish have great numbers of offspring and short lifespans. These traits can cause major changes in their abundance from year to year. If the abundance of forage fish increases or decreases significantly, the predators that eat them will also experience shifts in their population numbers.

Humpback whales are predators of herring. Many humpback whales migrate from Prince William Sound to Hawaii for the winter. Some humpback whales, however, stay in or near the Sound. During the winter, there is not much plankton for humpbacks to feed on, and fish like herring become a good alternative source of food for these whales.

Watch the video below to see how the predators of the pelagic hunt their herring prey.

VIDEO: Bait Ball Feast - BBC One

In late summer, the plankton bloom is at its height and vast shoals of herring gather to feed on it. Diving birds round the fish up into a bait ball and then a humpback whale roars in to scoop up the entire ball of herring in one huge mouthful. From "Nature's Great Events: The Great Feast" by BBC. (1:14)

The murres only attack from beneath, trapping the fish against the surface. But they push the herring within range of the gulls. It’s a feeding frenzy.

The table is set for the mightiest predator of them all: the humpbacks have reached their feeding grounds.

Scientists want to know the best way to estimate the numbers of specific fish species, such as herring. They get the data they need using a combination of aerial surveys, hydroacoustics, and various fish-capture techniques.

Check out the video below to hear Mayumi Arimitsu explain some of these techniques.

We have observers in a plane that are looking at schools of fish in the ocean very close to the shoreline. We do a couple of things. One is use hydroacoustics from the boat, and with basically a scientific fish finder we’re able to quantify the biomass and density and depth distribution of these different forage fish.

We also are trying to validate the aerial survey observations so we have a team in a skiff that are communicating with the pilot in the plane, and they are trying to catch what the observers in the plane are seeing.

Scientists working on the humpback whale monitoring project are trying to understand if the whales are having an impact on the recovery of herring populations in Prince William Sound. An important part of this project is maintaining an up-to-date humpback “fluke identification catalog,” a kind of “Who’s Who?” in the Gulf of Alaska whale world.

Watch the video below to learn about how scientists observe and photograph whales included in the fluke identification catalog.

VIDEO: Tracking Humpback Whales

John Moran describes how scientists are studying the importance of humpback whales in the Gulf of Alaska ecosystem. (2:08)

(Narrator) These small silver fish are Pacific herring, one of the many species being monitored by Gulf Watch Alaska. Scientists are monitoring their population for signs of recovery after the Exxon Valdez oil spill. They are also interested in other potential factors that could be affecting their recovery. One of these potential factors may be humpback whales.

(John Moran) We want to know if humpback whales are having an impact on the recovering herring population in Prince William Sound. Basically we want to know how many herring are whales eating, and is that important.

So the first thing we need to do is figure out how many whales are there, so we use Photo ID. All the whales have unique patterns on their flukes. When the whale dives it shows the underside of its fluke, and we’ll take a picture of that and that can identify the individual whale. So basically we get on the boat and we go look for whales. That the base of our research is getting the fluke IDs. And from that you can get a lot more information out of it.

We need to figure out what they’re eating, so we use the echo sounder on the boat, we’ll use nets and jigs, so we’ll see whatever prey is around the whale and try to catch that. Or if there’s any scales that slip out of their mouth, or any kind of sign of things on the surface, or fish jumping out of the whale’s mouth, we’ll try to document that.

And we also use biopsies. We have a cross bow or a rifle that takes a little blubber plug out of the whale. So we approach the whale and get a little sample, and from that we can use stable isotopes or fatty acids to get at what the diet’s been from that whale.

Humpbacks are kind of new players on the scene, they’re population was really low. In the late sixties & early seventies, there may have been 1,500-2,000 humpbacks in the North Pacific. And then there was this survey called the SPLASH survey that took place in 2006 that put the population at over 20,000. So that’s a huge increase.

It impacts managers. If you’re managing a herring fishery and you have these humpbacks population weren’t really there 20, 30, 40 years ago, you’ve got to account for these new predators, how many herring are they taking, it’s all important to know if you’re trying to manage a fishery. We haven’t had them there, so how they impact the ecosystem is going to be new to us.

Who is watching the Gulf?

Biomass (n): the amount of living matter in a given habitat (i.e. the weight of organisms per unit area, or the volume of organisms per unit of habitat)

Forage fish (n): small schooling fishes that feed on plankton and are eaten by larger predators

Hydroacoustics (n): the study of sound in water

Pelagic (adj): the open sea, away from the coast or seafloor

Trophic level (n): the position of an organism or species in a food web or food chain